High electrical resistivity thermostatic metal



April 24, 1962 c. F. ALBAN 3,030,699

HIGH ELECTRICAL REsIsTIvITY THERMOSTATIC METAL Filed Deo. 22, 1960 2 sf@ .9` TT E- l 477 T l l l /o o s0 o Je so o A207,

United States Patent O 3,030,699 HIGH ELECTRICAL RESlSTlVlTY THERMOSTATIC METAL Clarence F. Alban, Allen Park, Mich., assignor to W. M. Chace Company, Detroit, Mich., a corporation of Michigan Filed Dec. 22, 1960, Ser. No. 77,710 7 Claims. (Cl. 29-183.5)

This invention relates to laminated thermostatic metals having high electrical resistivity.

Laminated thermostatic metals comprise two or more laminae of metals or metal alloys having different tempera-ture co-eicients of expansion so that upon a change of temperature the metal will bend or ex due to the differential expansion of the laminae. The laminae are preferably joined by using known high temperature, pressure and time diffusion techniques in the solid state condition of the alloys to produce a high strength 'weld between the respective layers.

The known metal alloys suitable for use in laminated thermostatic metal elements, such as in bimetal strips, are limited in number since their performance over a broad range of temperature depends upon an optimum combination of a number of properties. Among these properties are: co-eicient of expansion, modulus of elasticity, elastic limit after cold rolling, ductility, metallurgical stability, strength at various temperatures and electrical resistivity. Electrical resistivity is particularly important in bimetals used in circuit breakers since such bimetals are directly heated by the electrical current flowing therethrough. There is an ever-present `demand for circuit breakers with lower ampere ratings, and hence there is a need `for a bimetal combination having as high an electrical resistivity as possible consistent with the other physical properties required for best performance.

Accordingly, it is an object of the present invention to provide laminated thermostatic metals having high electrical resistivities so that such metals may be heated directly by relatively small electrical currents without sacricing exivity or other established performance requirements.

Another object is to provide a bimetal lamination in which the thicknesses of the respective laminae are correlated With the alloy composition thereof to produce the highest possible electrical resistivity consistent with a usable co-ecient of deflection.

in the accompanying drawing:

FIG. 1 is an elevational View of a bimetal strip produced in accordance with the invention;

FIG. 2 is a graphic representation of thickness ratios of the bimetal strip plotted against the resistivity of the strip when utilizing an alloy combination of the invention; and

FIG. 3 is a graphic representation of the thickness ratios of the bimetal strip plotted against the ilexivity of the strip when utilizing the alloy combination of FIG. 2.

Referring in detail to the drawing, FIG. l shows a laminated thermostatic -rnetal element comprising a thermostatic bimetal strip A which may be incorporated in structures well known in the art to provide an arm rwhich deects under the influence of temperature changes therein. Bimetal A is constructed by using the aforementioned diffusion welding techniques whereby a high expanding alloy lamina 1 is bonded to a low expanding alloy lamina 2 at the adjacent faces thereof indicated at 3.

Such thermostatic bimetals when used in circuit breakers are commonly connected as a conducting element in the circuit and the heat developed by the current owing longitudinally therethrough is converted to mechanical work to open or close suitable contacts, as is well understood in the electrical art. The electrical power loss through bimetal A is represented by 12R, where I is current in amperes in bimetal A and R is the total resistance in ohms thereof. This power loss is converted to heat energy which determines the temperature of bimetal A for a given heat transfer condition. It will be evident that in order to develop the 12R effect to a usable level in circuit breakers, the electrical resistivity of the Ibimetal must be as high as possible'. The highest electrical resistivity hitherto known to applicant in a thermostatic bimetal is 850 ohms per circular mil foot. Such prior art material has established the lower limit on the ampere ratings `of circuit breakers.

In order to provide circuit breakers of lower ampere ratingsthe present invention correlates the composition and thickness of high expanding alloy lamina 1 with the composition and thickness of low expanding alloy lamina 2 to provide a combination having as much total electrical resistance as possible with adequate thermal deflection. It is known that electrical current is carried in parallel in laminae 11 and 2 of bimetal arm A as follows:

where Solving Equation 1 -algebraically gives:

TlTgT (2) R-tiTz-i-zi From this equation it will be seen that to obtain the highest value of total resistance R in the composite bimetal A, it is necessary that the lamina having the maximum thickness must also have the maximum resistivity. However, the ratio of thickness t1 of lamina 1 to the thickness t2 of lamina Z must be adequate to create the necessary forces in bimetal A during differential expansion to produce a usable amount of thermal dellection. In most thermostatic bimetals maximum thermal deflection is obtained by the following known relationship:

E1=modulus of elasticity of lamina 1 E2=modulus of elasticity of lamina 2 The present invention provi-des a laminated bimetal Awhich is a combination of particular high and low expanding alloys which together provide a high value of Valloy laminae are used in a preferred embodiment:

High expansion lamina 1 comprises an alloy of 72% Mn, 18% Cu, and 10% Ni having a thickness i1 of .9600 inch.

Low expansion `lamina 2 comprises an alloy of 17% Cr, 4% Al, and the balance Fe having a thickness t2 of .1830 inch.

Lamina 1 Lamina 2 Composition 72%0h911. 18% C11, 79% Fe, 17% CT,

/ i. 4% Al.

Expansion Coefficients from 14.94 l03lD F 5.00)(10-5!" F.

100 F. to 300 F,

Ohms/0.111,1. at 75 F 1 0S Modulus of Elasticity When the labove high land low expanding alloy laminae 1 and 2 are bonded together by the aforementioned diffusion welding technique, a combination is produced .having a ilexivity of 9.3X10-5 from 100 F. to .300 F. when tested using the American Society for Testing Materials Standard 13106-56. The electrical resistivity of this combination is 975 ohms per circular mil foot when tested at room temperature using ASTM .Standard B63- 49. Hence, this new thermostatic bimetal extends the upper limit of electrical resistivity by approximately 15% over that of the aforesaid prior art thermostatic bme'tal while achieving adequate exivity.

Referring to FIG. 2 the :relationship of the electrical resistivity of the above combined high and low expanding alloy laminae with respect to the ratios of the respective thicknesses t1, t2 thereof is shown Agraphically When R is computed in accordance with Equation 2. The respective thicknesses of the laminae 1 :and 2` ofthe preferred bimetal A are in the ratio 'of approximately 84% of high expansion lamina to 16% of low expansion lamina, which as seen in FIG. 2 gives a theoretical electrical resistivity value of 967 ohms/cmi. This is in substantial agreement with the tested value of `975 ohms/c mf.

FIG. 3 shows graphically the relationsip of the exivity of the above combined high and low expanding alloy laminae with the ratios of the respective thicknesses thi t2 thereof. When the values of ymodulus of elasticity for the high and low expanding laminae 1 rand 2 respectively are substituted in Equation 3 the ideal ratio for exivity is found to be 55.1% t1 to 44.9% t2, providing .a dexivity of 14.91 X 10-6. The intercept of these values is seen to be located at the peak of the curve of FIG. 3. this ratio of 55.l%/44.9% on the curve of FIG. 2 indicates a resistivity value of approximately 900.7 ohms/ 'c.m.f. Thus when the particular alloys described above are combined in accordance with the invention to form a thermostatic bimetal, the resulting bimetal exceeds in resistivity the prior art value of 850 ohms/cmi. even when the particular alloys are combined according .to the high activity thickness relationship of Equation 3.

While as shown in FIG. .2 the electrical resistivity of the bimetal can be increased `by increasing the percentage thickness of the high expanding alloy lamina 1 at the expense .of that of the low expanding alloy lamina 2, it

Plotting (Lamina 1) High (Lamina 2) Low Expanding Alloy Expanding Alloy 72%i14% Mn 17%i-9% Cr 18%=!;9% Cu 4% to 15% Al 10%;h5% Ni Balance lie From the foregoing it is to be understood that the invention provides two different alloy compositions which are combined for use as high and low expanding laminae to form a very high resistivity bimetal over a Wide range of thickness ratios. When the percentage thickness t1 of lamina 1 is increased from about 30% to about 84% the resistivity increases from about 850 to about 967 ohms/cmi. Without having the flexivity drop below 9.3 X10-5. Moreover, as the curve of FIG. 3 indicates it is possible to deviate substantially from the known preferred thickness ratio of Equation 3 by increasing the thickness of lamina 1 without thereby causing a corresponding substantial decrease in ilexivity. In 'other words the relationship is not linear, which permits a thickness ratio deviation up to that of 84-16 yof the preferred embodiment of the invention before further deviation causes a marked drop off in exivity.

I claim:

l. A laminated thermostatic metal including in combination, a lamina of high expanding alloy having a composition within the range of 5S percent to 86 percent manganese, 9 percent to 27 percent copper and 5 percent to 15 percent nickel, and a lamina of low expanding alloy having a composition Within the range ofV 59 percent to 88 percent iron, Y8 percent -to 26 percent of chromium and 4 percent to 15 percent aluminum, said laminae being bonded together for causing thermal deection in said thermostatic metal, the thickness of `said lamina of high expanding alloy being within the range of about 30 to about 84 Apercent of the total thickness of said laminae so that said laminae together provide a high value of electrical resistivity with a usable coefficient of deflection whereby said thermostatic -metalvrnay be heated for operation by passing a relatively small electric current through said laminae.

2. The combination set forth in claim l wherein the thickness of said lamina of high expanding alloy comprises substantially 84 percent of the 4total thickness of said laminae so that the velectrical resistivity of Vsaid thermostatic metal is maximized while obtaining a usable coeicient of deflection.

3. The combination set forth in claim 1 wherein said lamina of high expanding alloy is diiusion welded to said lamina of low expanding alloy by using high temperature, pressure and time diiusion techniques in the solid state condition of said alloys comprising said laminae to yproduce a high strength weld therebetween.

4. A laminated thermostatic metal comprising a first lamina of alloy having a composition of 72 percent manganese, 18 percent copper and l0 percent nickel, and a Vsecond lamina of alloy having a composition of 79 per- 5 for operation by passing a relatively small electric current through said larninae.

5. The combination set forth in claim 4 wherein the thickness of said first lamina comprises substantially 84 percent of the total thickness of said laminae so that the electrical resistivity of said bimetal is maximized while obtaining a usable coefiicient of deflection.

6. The combination set forth in claim 4 wherein said thicknesses of said irst and second laminae respectively equals .9600 inch and .1830 inch to provide a cornposite bimetal ingot having a exivity of 9.3 X10*6 from approximately 100 to 300 degrees F. and having an electrical resistivity of approximately 975 ohms per circular mil foot at approximately 75 degrees F.

7. The combination set forth in claim 4 wherein said laminae are diiusion welded together by using high temperature, pressure and time diffusion techniques in the solid state condition of said alloys comprising said laminae to produce a high strength weld therebetween.

References Cited in the le of this patent UNITED STATES PATENTS 2,234,748 Dean Mar. 11, 1941 2,317,018 Alban Apr. 20, 1943 2,403,895 Alban July 16, 1946 

1. A LAMINATED THERMOSTATIC METAL INCLUDING IN COMBINATION, A LAMINA OF HIGH EXPANDING ALLOY HAVING A COMPOSITION WITHIN THE RANGE OF 58 PERCENT OT 86 PERCENT MANGANESE, 9 PERCENT TO 27 PERCENT COPPER AND 5 PERCENT TO 15 PERCENT NICKEL, AND LAMINA OF LOW EXPANDING ALLOY HAVING A COMPOSITION WITHIN THE RANGE PERCENT TO 88 PERCENT IRON, 8 PERCENT TO 26 PERCENT OF CHROMIUM AND 4 PERCENT TO 15 PERCENT ALUMINUM, SAID LAMINATE BEING BONDED TOGETHER FOR CAUSING THERMAL DEFLECTION IN SAID THERMOSTATIC METAL, THE THICKNESS OF SAID LAMINA OF HIGH EXPANDING ALLOY BEING WITHIN THE RANGE OF ABOUT 30 TO ABOUT 84 PERCENT OF THE TOTAL THICKNESS OF SAID LAMINAE SO THAT SAID LAMINAE TOGETHER PROVIDE A HIGH VALUE OF ELECTRICAL RESISTIVITY WITH A USABLE COEFFICIENT OF DEFLECTION WHEREBY SAID THEREMOSTATIC METAL MAY BE HEATED FOR OPERATION BY PASSING A RELATIVELY SMALL ELECTRIC CURRENT THROUGH SAID LAMINAE. 